628 



INVERTEBRATE MORPHOLOOT. 



of the test ; tlie cavity it contains is continuous with the body- 

 coelom and is therefore lined by mesoderm, and is divided 

 into two compartments by a longitudinal 

 partition which may be traced back to 

 its origin from the posterior wall of the 

 branchial sac of the original individual. 

 Since ectodermal tissue lies between the 

 mesoderm and the inner surface of the 

 test, the stolon contains portions of all 

 three germ-layers, and a portion of each 

 passes into each bud (i) as it arises. 

 The first indication of a bud is a slight 

 wartlike elevation of the wall of the 

 stolon which increases in size, its cavity 

 being a diverticulum of the stolon-coelom. 

 286.— Portion OF A Into the elevation a process (ew) of the 

 Stolon of -P^'-oPf™ gndodermal stolon-partition extends, and, 



(after Kowalewsky from _ in ^■1 ■, -, ■ 



KoKscHELT and heidkr). forming a hollow saclike body, gives 

 rise to the digestive tract of the bud. 

 The various layers give rise to their 

 respective organs with one exception, 

 and that is that the atrial walls, the man- 

 tle, arise from the endodermal branchial sac as diverticula 

 which unite together, the atrial cavity being thus lined 

 throughout with endoderm. Such anomalies are not infre- 

 quent in the Urochorda, and indicate a necessity for further 

 study of the nature of the germ-layers in these forms. 



The simple non-budding forms are quite numerous. Com- 

 mon genera are Molgula, Cynthia in which the test has a 

 leathery consistency owing to the fibrillar character of the 

 matrix, and Boltenia, a stalked form. 



Fig. 



6 = bud. 

 br = branch of stolon. 

 ec = ectoderm. 

 en — endoderm. 



3. Suborder Ascidice compositcB. 



All the members of this order reproduce by budding in 

 some form or other, and differ from such forms as ClaveHina 

 in that all the individuals remain imbedded in a common test 

 whether or not they remain in organic connection with one 

 another. The group seems to be a somewhat composite one, 



